Hey there! Let's walk through the most common foundational mistakes that trigger that annoying Uncaught SyntaxError: Unexpected identifier error—since you mentioned it's popping up both in regular code lines and class function declarations, these are the top culprits to check:

• Forgot commas between object properties
  This is super easy to slip up on. When defining an object, every property (including method definitions) needs a trailing comma (except the last one). If you skip it, JavaScript gets confused when it hits the next identifier:

  // ❌ Wrong (missing comma after "name")
  const user = {
    name: "Alice"
    greet() { console.log("Hey there!") }
  }
  
  // ✅ Right
  const user = {
    name: "Alice",
    greet() { console.log("Hey there!") }
  }
  

• Using function keyword in class methods
  Class methods have a special syntax—you don't need to prefix them with function. Adding it throws off JavaScript, which expects a method name (identifier) instead of the function keyword:

  // ❌ Wrong
  class Calculator {
    function add(a, b) { // "function" is unexpected here
      return a + b;
    }
  }
  
  // ✅ Right
  class Calculator {
    add(a, b) {
      return a + b;
    }
  }
  

• Typos or using reserved words as identifiers
  If you misspell a keyword (like writing cost instead of const) or use a JavaScript reserved word (like class, function, let) as a variable/method name, JavaScript will flag the following identifier as unexpected:

  // ❌ Wrong (misspelled keyword)
  cost userName = "Bob";
  
  // ❌ Wrong (using reserved word)
  let class = "Beginner JS";
  
  // ✅ Right
  const userName = "Bob";
  let className = "Beginner JS";
  

• Automatic Semicolon Insertion (ASI) gotchas
  Skipping semicolons can sometimes cause JavaScript to merge lines incorrectly, leading to unexpected identifiers. This often happens when the next line starts with an array or object literal:

  // ❌ Wrong
  const num = 10
  [1, 2, 3].forEach(n => console.log(n)) // JS tries to run num[1,2,3], which is invalid
  
  // ✅ Right
  const num = 10;
  [1, 2, 3].forEach(n => console.log(n));
  

• Mismatched template literal syntax
  If you're using template strings and forget the closing } in an interpolation, JavaScript will throw this error when it hits the next unexpected identifier:

  // ❌ Wrong (missing closing })
  const greeting = `Hello, ${userName`;
  
  // ✅ Right
  const greeting = `Hello, ${userName}`;
  

If you can share a small snippet of the exact code where the error is occurring, we can zero in on the exact issue even faster. But chances are, it's one of these common foundational missteps!

内容的提问来源于stack exchange，提问作者DarkCruncher